1. Field of the Invention
The invention relates to a method for defoaming and/or for preventing the foaming of aqueous compositions comprising surfactants, on the basis of siloxanes.
2. Description of the Related Art
In many liquid systems, more particularly aqueous systems, which include surface-active compounds either as wanted or else unwanted constituents, problems due to foaming may occur if these systems are brought into more or less intense contact with gaseous substances, as for example when gasifying waste waters, when intensively stirring liquids, in distillation, washing, or coloring operations, or during dispensing procedures.
Controlling this foam may be accomplished mechanically or through the addition of defoamers. In this context, siloxane-based defoamers have proven themselves particularly useful.
Defoamers based on siloxanes are prepared according to U.S. Pat. No. 3,383,327 A, for example, by heating hydrophilic silica in polydimethylsiloxanes. The activity of such defoamers can be improved by using basic catalysts (U.S. Pat. No. 3,560,401 A).
Proposed alongside polydimethylsiloxanes are organopolysiloxanes which, as well as methyl groups, also carry other relatively long-chain, aliphatic or aromatic hydrocarbon groups on the silicon. Oftentimes the aim of introducing long alkyl groups is to enhance compatibility with mineral oils which may be present in defoamer compositions. Furthermore, defoamer formulations with organopolysiloxanes having relatively long alkyl groups or phenyl groups exhibit an improved defoamer effect by comparison with defoamer formulations comprising polydimethylsiloxanes. Defoamer formulations of these kinds are described, for example, in EP 578 424 A2, where organopolysiloxanes having alkyl groups with 9 to 35 carbon atoms are used, and in EP 1 075 863 and EP 1 075 864, where organopolysiloxanes with at least one Si-bonded substituent of the formula X-Ph (X=divalent alkylene group, Ph=phenyl group) are used, and in U.S. Pat. No. 7,619,043 B2, where organopolysiloxanes with aromatic hydrocarbon radicals are used, or in US 2009/0137446 A1.
However, in strongly foaming systems, especially those rich in anionic surfactants, the defoamer formulations produced in accordance with the prior art do not always display adequate activity.
Attempts are therefore being made to boost the defoamer activity by combination of customary silicone defoamers with other materials, including waxlike non-silicone materials. Accordingly, in EP 266 863 A1, alkylphosphoric acid or salts thereof are added as antifoam promoters. In EP 1 118 655 A1, unsaturated alcohols in combination with silicone antifoams are described for supporting the ability of the silicone antifoams. U.S. Pat. No. 7,632,890 B2 describes the addition of relatively strongly polar components preferably possessing an active hydrogen atom, examples being fatty alcohols, ethoxylated fatty acids, or ethoxylated alkylphenols, or else partial esters of polyols, such as monoesters of glycerol and fatty acids. Together with waxlike addition compounds which comprise an apolar polyol ester, these components are combined with the silicone defoamers.
A problem of the nonsilicone materials used is their compatibility with the silicone defoamers whose activity is to be improved.